In recent years, a significant industry has developed which involves the application of customer-selected designs, messages, illustrations, and the like (referred to collectively hereinafter as “images”) on articles, such as T shirts, sweat shirts, leather goods, and the like. These images may be commercially available products tailored for a specific end-use and printed on a release or transfer paper, or the customer may generate the images on a heat transfer paper. The images are transferred to the article by means of heat and pressure, after which the release or transfer paper is removed.
Much effort has been directed at generally improving the transferability of an image-bearing laminate (coating) to a substrate. For example, an improved peelable heat transfer material has been described in U.S. Pat. No. 5,798,179, which allows removal of the base sheet immediately after transfer of the image-bearing laminate (“hot peelable heat transfer material”) or some time thereafter when the laminate has cooled (“cold peelable heat transfer material”). Moreover, additional effort has been directed to improving the crack resistance and washability of the transferred laminate in applications where the transferred laminate must be able to withstand multiple wash cycles and normal “wear and tear” without cracking or fading.
Heat transfer papers generally are sold in standard printer paper sizes, for example, 8.5 inches by 11 inches. Graphic images are produced on the transferable surface or coating of the heat transfer paper by any of a variety of means, for example, by ink-jet printer, laser-color copier, other toner-based printers and copiers, and so forth. The image and the transferable surface are then transferred to a substrate such as, for example, a cotton T-shirt. In most instances, transfer of the transfer coating to areas of the articles which have no image is necessary due to the nature of the papers and processes employed, but it is not helpful or desirable because the transfer coatings can stiffen the substrates, make them less porous and make them less able to absorb moisture.
Thus, it is desirable that the transferable surface only transfer in those areas where there is an image, reducing the overall area of the substrate that is coated with the transferable coating. Some papers have been developed that are “weedable”, that is, portions of the transferable coating can be removed from the heat transfer paper prior to the transfer to the substrate. Weeding involves cutting around the printed areas and removing the coating from the extraneous nonprinted areas. However, such weeding processes can be difficult to perform, especially around intricate graphic designs.
In recent years, a product called Image Clip® Laser Light Heat Transfer Paper commercially available from Neenah Paper, Inc. (Alpharetta, Ga.) has been employed. This product extends the range of images which can be weeded to fine detailed images and simplifies the weeding process. In this process, a special imaging sheet (first transfer sheet) is printed with a toner image and a matched transfer sheet (second transfer sheet) is applied to the first transfer sheet with heat and pressure, resulting in transfer of the second transfer sheet coating to the image of the first transfer sheet when the papers are separated. Little or no coating is transferred in the non-imaged areas. Then, in a second transfer step, the coated image is transferred to a desired substrate. The first transfer step is preferably done at a lower temperature than the second transfer step to avoid transfer in the non-imaged areas. Such processes utilize adhesive properties of the toner to achieve transfer in the first transfer step.
However, when printing light colored images or images containing areas of light coloring and/or shading, insufficient toner could be applied to provide sufficient adhesion for heat transfer purposes, creating a faded or void spot in the image, when using conventional toner printers.
In the case of transfers to dark colored fabrics, it is desirable to have the ability to transfer white, opaque images as well as colored images, and especially to transfer images which contain both colored portions and white portions. Recently a dark fabric transfer paper which is capable of transferring a weeded opaque layer along with a colored image has become available under the name Image Clip® Laser Dark Heat Transfer Paper from Neenah Paper, Inc. (Alpharetta, Ga.). This product functions in essentially the same manner as the Image Clip® Laser Light Heat Transfer Paper. Besides having a white, opaque layer which is transferred with the image, this product also has a light transfer coating on the transfer paper which bears the image. This coating transfers to the substrate along with the image and the coating from the second transfer sheet in the second transfer step. However, this product, like the Image Clip® Laser Light Heat Transfer Paper, also depends on toners for the adhesive in the first transfer step. Thus, it is only possible to transfer colored images. Also, images containing a very light toner coverage might not transfer completely, as is the case for the Image Clip® Laser Light Heat Transfer Paper product.
Therefore, there remains a need in the art for improved heat transfer papers and methods of application. Desirably, the papers and methods provide good image appearance and durability.